Transponder selecting methods also referred to as anti-collision methods, are used for example in contactless identification systems. Such systems are known as radio frequency identification systems (RFID). Such systems conventionally comprise a base station or a so-called reading device and a plurality of transponders or remote sensors equipped with transponders. All of these transponders are simultaneously present in the communication range of the base station. In order to make sure that a data transmission takes place only between the base station and one transponder or between the base station and a group of transponders, it is necessary to first perform a selection method prior to any data transmission.
As a rule, conventionally a distinction is made between stochastic and deterministic selection methods. A detailed description of deterministic and stochastic selection methods can be found, for example in the RFID-Manual, Third Edition, author Klaus Finkenzeller, publisher Hanser, 2002, see particularly Chapter 7.2, entitled “Multiple Access Methods”.
Stochastic methods do not require, in contrast to deterministic methods, an unambiguous so-called unique identification U-ID. The structure of such unique identification is, for example described in the Standard Sheet ISO 15963. An allocation of such U-IDs is performed, among others, by different organizations that are independent of the manufacturer of such base station transponder systems. Such organizations are, for example the EAN/UCC or the IATA. The allocation may, however, also be made by the manufacturer of the system. As a result, it is not always possible to assure the unambiguity of the U-ID in open systems in which transponders of different manufacturers may be within the communication range of a base station. Stochastic identification methods, however, make a selection possible even in systems without an unambiguous identification allocation. Examples of such stochastic methods are, for example the so-called ALOHA-method, the slot based or slotted ALOHA-method, and the dynamic slot based ALOHA-method.
The ALOHA-method is a transponder controlled, stochastic method in which the transponders transmit the data to be transmitted in a time sequence manner. The time sequence or time staggering is adjusted, as a rule, on the basis of a random number generated in the transponder. A so-called collision occurs, when several transponders transmit a marker within the same time slot. Thus, generally such a collision prevents that the base station can receive the transmitted data without any errors.
When comparing a simple ALOHA-method with a time slot based ALOHA-method the collision probability is substantially reduced in the time slot based ALOHA-method. The slot based ALOHA method is a stochastic method controlled by the base station. In such a method the transponders are active only at defined, synchronous points of time, whereby “active” means that data transmission begins. For this purpose the base station generates numbered time slots and each transponder generates a random transponder number, whereby each transponder whose random number corresponds to the number of a time slot, transmits data or a marker during this time slot to the base station. As a rule, the base station transmits a command to the transponders to initiate the selection procedure. The command also indicates the begin of a selection procedure. Upon receipt of the command, the respective random numbers are stored in a memory in the corresponding transponder. These random numbers are, for example generated in the respective transponder prior to any receipt of a command. If only one transponder transmits a marker within one time slot, that transponder is selected within the time slot or this transponder can be selected by the base station by transmitting from the base station an acknowledgment signal. At that point the base station can, for example, perform recording and/or reading operations with regard to the selected transponder. If several transponders transmit a marker within the same time slot, a so-called collision occurs. Depending on the bit coding, a base station can recognize such collisions either directly or with a certain delay, whereupon the respective time slot is ignored and attempts are made to find time slots in which no collision occurs or a new selection procedure is started by sending a new respective command to the transponders. Since the transponders produce or store new random numbers, the probability exists that in the separate procedure no collision will occur.
The collision probability depends on the number of the transponders present in the communication range of the base station and on the number of the provided time slots. A static determination of the number of time slots can lead to problems because the number of transponders present within a communication range of the base station may vary substantially. If the number of time slots is too small, the collision probability is substantial. On the other hand, if the number of time slots is too large, the result is too many time slots in which no transponder transmits data. In both instances, the time required for performing the selection procedure increases substantially. In order to achieve an optimal throughput, so to speak, the number of time slots during which the transponder transmit data, should correspond approximately to the number of transponders within the communication range of the base station.
The dynamic time slot based ALOHA-method aims at solving the above problem by enabling the base station to control the number of available time slots. For example, the base station can initiate a selection procedure with a small number of time slots. If collisions occur frequently because of the limited number of time slots, the base station can initiate a new selection procedure in which the number of time slots is increased, whereby the collision probability decreases. However, the time required for the performance of such a dynamic method or procedure is relatively large because the adjustment or selection of the optimal number of times slots can take up a substantial length of time. Another drawback of the dynamic slot based ALOHA-method is seen in the high effort and expense for the circuit arrangement required due to the method's complexity. Different methods for generating a random number are known for use in the stochastic procedure. For example the time period between a reset of the transponder and the point of time at which a first marker is received, can serve as the basis for calculating the random number. Other methods link numbers from two different memory areas of the transponder with each other in order to ascertain the random number. In that case, the selection procedure may be refined by additionally using a received data in the calculation of the random number. Other methods use a linear feedback controlled shift register for producing the random number. Such a shift register may, for example be operated by an inaccurate clock signal.
Deterministic selection procedures generally rely on a binary search method or on a so-called binary search algorithm. These deterministic search procedures assume that an unambiguous identification bit sequence is allocated to each transponder in the communication range of the base station. The identification bit sequence is customarily produced by the unique identification U-ID. Such a unique identification is allocated to the transponders, for example when the transponders are manufactured. The unambiguity of the U-ID can, however, not always be guaranteed due to the multitude of U-ID specifications in open systems that are not limited to a certain transponder type.
U.S. Pat. No. 5,856,788 describes a selection procedure in which the transponders are selected on the basis of a bit-by-bit comparing of the transponder's unambiguous, statically allocated identification bit sequence with a selection bit sequence. In the method according to U.S. Pat. No. 5,856,788 the selection is made by a semi-duplex procedure in which each transponder transmits a bit of its identification bit sequence and the base station subsequently transmits a selection bit based on the transmitted bit values. The transponders compare the respective bit of their identification bit sequence with the corresponding selection bit on the basis of a comparing criterion. The comparing criterion is the so-called “equal operator”. More specifically, those transponders remain activated, which have in their identification bit sequence a respective bit that corresponds with the selection bit. Due to the semi-duplex method relatively much time is necessary until a transponder is selected.
In a publication “ISO WD 18000-6 Mode 3”, published Feb. 1, 2002, a selection method is described in which the selection takes place also based on an unambiguous identification bit sequence. This known selection method operates as a full-duplex method, whereby the selection duration is reduced. The transponders are selected in that method in accordance with their unambiguous statically preassigned identification bit sequence.